Transmission devices known from practice for use in mobile working machines such as construction site machines and agricultural machines are increasingly made in the form of continuously power branched transmissions. Depending on the application and intended purpose, these are designed as so-termed primary coupled or secondary coupled transmissions, wherein a primary or secondary coupling indicates the respective position of the rotational speed distribution in the transmission, either at the input or in the area of the transmission output.
Continuously power branched transmission devices of construction site and agricultural machines are often hydrostatically power branched transmissions, in which part of the power passes along a mechanical path and the other part of the power is passed by virtue of the power branching to a hydrostatic variator, in the area of which the continuous adjustment of the transmission ratio takes place. After the power branching, the power components that have passed by way of the two paths are brought together again and passed on toward the drive output.
From DE 601 33 609 T2 and DE 197 49 074 02 drive-train concepts are known, which are made with continuously power branched transmission devices in which, in each case, an electrical variator is provided.
FIG. 1 shows a vehicle drive-train 1A with a transmission device 1 known from the prior art, which comprises an electrical variator 2 for the continuous variation of a transmission ratio of the transmission device 1. A planetary gear unit 3 of the transmission device 1 is coupled with a transmission input shaft 4, a shaft 29 of the variator 2 and a change-speed transmission 5, in the area of which, in combination with the variator 2, in each case four transmission ratio ranges for forward and for reverse driving can be obtained. Within the four transmission ratio ranges, the transmission ratio of the transmission device 1 can in each case be adjusted continuously by means of the variator 2, and for that purpose the variator 2 is also connected in the area of a second shaft 21 to the transmission input shaft 4. The variator 2 comprises a first electric machine 7A and a second electric machine 7B, each of which can be operated as a motor and as a generator.
Besides the planetary gear unit 3, the change-speed gearbox 5 comprises three further planetary gearsets 8 to 10, whose shafts can be coupled to one another by way of five frictional shifting elements 11 to 15 in the manner described in more detain later in order to obtain the four transmission ratio ranges, and all of which are arranged coaxially on the transmission input shaft 4.
On the transmission output side of the change-speed gearbox 5, two further frictional shifting elements 17 and 18 are provided, such that when the frictional shifting element 17 is closed forward driving is possible, whereas when the frictional shifting element 18 is closed a vehicle constructed with the vehicle drive-train 1A and the transmission device 1 can be operated in the reverse driving direction.
In the area of a further frictional shifting element 19 the transmission input shaft 4 connected to a drive engine 20 of the vehicle drive-train can be coupled to a drive input shaft 22 by means of which auxiliary power takeoff drives of a vehicle, preferably one in the form of a tractor or the like, can be powered.
In this case the first planetary gear unit 3 is functionally connected in the area of a sun gear 23 to the shaft 29 of the variator 2, while a planetary carrier 24 of the first planetary gear unit 3 is attached in a rotationally fixed manner to a ring gear 25 of the second planetary gearset 8 and to a planetary carrier 26 of the third planetary gearset 9. A ring gear 27 of the first planetary gear unit 3 is coupled to a planetary carrier 28 of the second planetary gearset 8. A sun gear 30 of the second planetary gearset 8 is connected rotationally fixed to a sun gear 31 of the third planetary gearset 9. A ring gear 32 of the third planetary gearset 9 can be coupled by the first frictional shifting element 11 to a sun gear 33 of the fourth planetary gearset 10, whereas the sun gears 30 and 31 of the planetary gearsets 8 and 9 can be brought into functional connection with the sun gear 33 of the fourth planetary gearset 10 by means of the second frictional shifting element 12. The planetary carrier 24 of the first planetary gear unit 3, the ring gear 25 of the second planetary gearset 8 and the planetary carrier 26 of the third planetary gearset 9 can be brought into functional connection, by means of the third frictional shifting element 13, with a planetary carrier 34 of the fourth planetary gearset 10, which is connected in a rotationally fixed manner to a common outer disk carrier 35 of the frictional shifting elements 17 and 18. A shaft in the form of a ring gear 16 of the fourth planetary gearset 10 can be made rotationally fixed by means of the frictional shifting element 15 in the form of a brake.
An inner disk carrier 36 of the frictional shifting elements 17 is connected in a rotationally fixed manner to a gearwheel 37, which meshes with a spur gear 38 connected in a rotationally fixed manner to a transmission output shaft 39 of the transmission device 1. In addition, an inner disk carrier 40 of the frictional shifting element 18 is coupled to a spur gear 41, which by way of an intermediate wheel 42 that meshes with it, engages with a further spur gear also connected in a rotationally fixed manner to the transmission output shaft 39.
The transmission output shaft 39 is functionally connected by way of a bevel gear, in a manner not illustrated in more detail, to an axle differential transmission device of a driven vehicle axle 44 forming a rear axle of the vehicle, and can be coupled by a further frictional shifting element to a second driven vehicle axle.
To obtain the first transmission ratio range for forward driving or for reverse driving, the frictional shifting elements 11 and 15 must be changed to their closed condition whereas the other frictional shifting elements 12, 13 and 14 must be open. If a change starting from the first to the second transmission ratio range is called for, the first frictional shifting element 11 has to be opened and the second frictional shifting element 12 closed, while the frictional shifting element 15 remains in its closed condition and the shifting elements 13 and 14 remain open.
Starting from the second transmission ratio range, if a change to the third transmission ratio range is called for, the frictional shifting element 15 has to be opened and the frictional shifting element 13 closed, whereas for a change from the third to the fourth transmission ratio range the frictional shifting element 13 has to be opened and the further frictional shifting element 14 changed to its closed operating condition while at the same time the frictional shifting element 12 is closed.
When the first transmission ratio range is engaged, a vehicle constructed with the vehicle drive-train 1A according to FIG. 1 can be operated in a speed range from 0 to around 6 km/h. When the second transmission ratio range is engaged, it can be operated in a speed range from around 6 km/h to around 13.5 km/h, and when the third transmission ratio range is engaged a speed range for operating the vehicle between around 13.5 and around 27 km/h is available. With the transmission device 1 according to FIG. 1, if the fourth transmission ratio range is engaged the vehicle can be operated within a speed range of around 27 to around 58 km/h.
In this case, however, it is problematic that by virtue of the design layout, the maximum transmissible power in the area of the electric machines 7A and 7B in some operating ranges of the vehicle drive-train 1A or speed ranges of a vehicle constructed with the vehicle drive-train 1, restricts a transmission output torque and hence the traction force in the area of the wheels of the driven vehicle axles 44. This occurs particularly in operating ranges within which idle power is circulating, and causes high loading in the area of the electric machines 7A and 7B.
If a vehicle constructed with the vehicle drive-train 1A is operated in operating ranges precisely around a speed at which a shaft of the planetary gear unit 3 connected thereto must be brought to rest by one of the electric machines 7A or 7B, the electric machine 7A or 7B has to be operated as a generator so that in the area of the electric machine the rotational speed in some circumstances has to be controlled to zero, even for a prolonged operating period. Particularly when the electric machine 7A or 7B is controlled without sensors, the regulation of very low speeds down to zero is very complicated.